In a conventional monitoring apparatus in which the removal of a high voltage variation is important, a high voltage generating circuit and a horizontal deflection circuit are provided separately and a stabilizing circuit is also provided in the high voltage generating circuit, although these arrangements are complicated. When considering cost reduction and power economy, however, a high voltage circuit of a so-called flyback transformer type in which a high voltage is generated by a flyback transformer serving both as the high voltage generating circuit and the horizontal deflection circuit, is superior to the above circuit arrangement.
FIG. 1 shows an arrangement of a conventional high voltage circuit of the flyback transformer type, which takes no measure for voltage stabilizing. In the circuit, when a drive pulse is applied to the base of a horizontal output transistor Q1, it produces a horizontal deflection pulse. During the flyback period of the horizontal deflection pulse, a collector pulse voltage Vcp is produced. After the voltage Vcp is boosted to a high voltage by a flyback transformer FBT, the boosted voltage is further rectified by a high voltage rectifying diode D1 to obtain an output of a DC voltage HV. Here, D2 designates a dumper diode, Ct a resonating capacitor, Ly a horizontal deflection coil, and Cs a DC blocking capacitor. A DC voltage Vcc is supplied to the terminal of the primary side of the flyback transformer FBT.
The collector pulse voltage Vcp is generally expressed by ##EQU1## where T.sub.H is the duration of one period of a horizontal deflection frequency, and T.sub.R is a flyback period.
When a load of the high voltage circuit is fixed, the DC high voltage HV is proportional to the collector pulse voltage Vcp. Accordingly, when the high voltage load varies and the DC high voltage HV decreases, if the flyback period T.sub.R in equation (1) is reduced, following proportionally the decrease of the DC high voltage HV, the collector pulse voltage Vcp increases and the DC high voltage HV also rises.
The flyback period T.sub.R is generally expressed by the following equation: ##EQU2## where Lyi is an inductance of the deflection coil Ly, and Ctc is a capacitance of a resonating capacitor Ct.
As seen from the above equations (1) and (2), if the inductance Lyi of the deflection coil Ly and/or the capacitance Ctc of the resonating capacitor Ctc is changed according to a variation of the high voltage load, the DC high voltage HV can be stabilized.
A conventional high voltage stabilizing circuit of the reactor type in which the inductance Lyi is changed has many disadvantages including poor transient response, heavy weight, large size, high manufacturing cost and so on. To overcome those disadvantages, another high voltage stabilizing circuit which controls the collector pulse voltage Vcp by changing a capacitance Ctc of the resonating capacitor Ct, is disclosed in Japanese Patent Application Laid-open No. 56-134879. In such a conventional stabilizing circuit, however, the control operation is continuously performed during periods other than the flyback period T.sub.R. As a result, the circuit loss is large, and hence the power economy is not attained. Additionally, the circuit operation is not stable due to heat generation in the circuit.